There are a couple of reasons men may experience decline in testosterone levels. The most common reason is the inevitable process of aging, where the testes simply don’t produce the same amount of testosterone as they previously did.1-3

Another reason is the increased activity of SHBG (sex hormone binding globulin), a testosterone binding protein that has been shown to increase with age.1-3

Testosterone levels can also be affected by the activity of the enzyme aromatase. This enzyme converts testosterone into estradiol (estrogen). Estrogen is necessary for male health, but an excess conversion can deprive the body of needed testosterone and leave it in an estrogen-dominant state.